Metal-oxide (MOx) particles, such as TiO2 and ZnO, serve many functions in polymeric materials. Traditionally, they have been used as pigments to enhance the appearance and improve the durability of polymeric products, and usually they have been considered to be inert. As nanosized particles, these materials exhibit broad band UV absorption, a benefit that has long been exploited in sunscreen applications.
Metal-oxide nanoparticles are also useful for a large variety of more sophisticated applications that become possible when uniform nanoparticles become available. Such applications include uses in catalysis, as sensors, optoelectronic materials and in environmental remediation. Controlled synthesis of metal oxide nanoparticles is essential for such applications, where uniformity of size and shape of MOx nanoparticles is needed to improve their usefulness.
The conventional prior art preparation techniques for MOx NPs typically use organometallic precursors to form NPs with diameters greater than 2 nm. With the prior art methodology, only larger structures such as nanorods, nanotubes, nanoneedles, and nanowires have been reported in literature.
For example, Gu et al. (2002) synthesized tungsten oxide (WOx) nanowires (5 nm×500 nm) by hydrogen treatment of W substrates at 700° C. Lee et al. (2003) synthesized WOx nanorods (3.5 nm×31 nm) using an organic precursor and an oxidation agent a 270° C. Hudson et al. (2003) synthesized WOx nanoneedles (10-60 nm) by pyrolysis of an acidified precursor in a copolymer at 900° C. Zhang et al. (2004) synthesized WOx nanorods (20 nm×1-2 μm) by electrochemical etching on W filaments; and Seo et al. (2005) synthesized WOx nanorods (4.5 nm×30 nm) using WCl4 and a mixture of two organic surfactants (oleylamine and oleic acid) at 350° C.
All the above methods failed to synthesize ultrasmall tungsten oxide nanoparticles because of (1) the W precursor used (2) the surfactant chosen and/or (3) the reaction conditions.
Similar to tungsten oxide, no one has reported the synthesis of ultrasmall vanadium oxide (VOx) nanoparticles. Lutta et al. (2005) reported the synthesis of VOx nanofibers (140 nm×1 μm) using ammonium vanadate and acetic acid after heating>250° C. Viswanathamurthi et al. (2003) reported the synthesis of VOx nanofibers prepared by electrospinning using vanadium sol and polyvinylacetate. Spahr et al. (1998 and 1999) reported the synthesis of VOx nanotubes (50 nm×100 nm) via hydrolysis using vanadium oxide triisopropoxide and hexadecylamine (180° C. and 10 bar). Muhr et al. (2000) reported the synthesis of VOx nanotubes (15-100 nm×5-50 nm) via hydrolysis using alkyl amines and vanadium alkoxide; and Niederberger et al. (2000) reported the synthesis of VOx nanotubes (60-100 nm×1-3 μm) via a 2-step hydrolysis-hydrothermal treatment using VOCl3 and V2O5.
Likewise, no one has demonstrated an ability to synthesizing ultrasmall molybdenum oxide (MoOx) nanoparticles. Phuruangrat et al. (2009) reported the synthesis of MoOx nanowires (50 nm×10 μm) using ammonium metamolybdate and CTAB using a microwave-assisted hydrothermal process. Zach et al. (2000) reported the synthesis of MoOx nanowires (15-1000 nm×500 nm) by electrodeposition of MoOx on graphite. Zhou et al. (2003) also reported the synthesis of MoOx nanowires (50-120 nm×4 μm) by heating Mo at 1100° C. on a silica substrate. Du et al. (2008) reported the synthesis of MoOx nanospheres (25-75 nm) using 3-mercaptopropyltrimethoxysilane using ultrasonic irradiation, and Niederberger et al. (2001) reported the synthesis of MoOx nanofibers (50-150 nm×15 μm) using sodium molybdate and dodecyl- and hexadecylamine in a template-directed approach.
As with tungsten, all the above methods for synthesizing vanadium and molybdenum nanoparticles were incapable of synthesizing ultrasmall nanoparticles due to thermodynamic or synthetic limitations.
Therefore, what is needed in the art is a method of making relatively uniform MOx nanoparticles of size less than 5 nm, preferably less than 2 nm, with narrow size distribution and control over the morphology and metal content of the nanoparticles.